Use -buildmode=pie (position independence executable)

[tonymeehan]

Describe the enhancement:

Add a buildmode flag to enable our Go binaries to support ASLR. This should be included in Elastic Agent, Beats and Fleet Server.

A few years ago Golang added support for -buildmode=pie. This enables Go binaries to take advantage of ASLR and helps us mitigate the potential for someone to create a reliable exploit if we one day discover a vulnerability. There are a few resources online to learn more about why ASLR is important, like this blog.

I noticed a few other Go projects have successfully added this, like Cloud Foundry's go-buildpack.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[mostlyjason]

Its possible that golang 1.15 already adds ASLR support by default. @urso offered to double check

[urso]

Checking the release notes, it seems position independent code is enabled by default for Windows and Linux targets. Anyways, I made the buildmode=pie more explicit for platforms that support this feature (including AIX and macOS) in #24964. Let's see what our build environment thinks about it.